Since the discovery of the vulcanization process in the mid-1800's, there has been much interest in recycling of vulcanized rubber, such as discarded tires and tire factory wastes.
One method for recycling tire rubber involves shredding tires and reforming the shredded tire into low specification materials such as rubber mats, rubber blocks and vehicle mud flaps. Shredded tire rubber has also been suggested as an additive to road asphalt. Another method for recycling tire rubber involves subjecting tires to a pyrolysis reaction to produce a high API gravity, low sulfur, aromatic oil which is useful as a fuel. However, these recycling processes are generally not economic and produce a low quality rubber product. Accordingly, discarded tires are currently being stockpiled and/or landfilled. This is not a suitable solution as evidenced by the recent large tire fire in Hagersville, Ontario, Canada. Clearly, discarded tires represent a considerable environmental liability.
In the vulcanization process, rubber polymer is cross-linked with sulfur, usually with the application of heat. Unfortunately, cross-linked rubber polymer cannot be reclaimed into a useful product merely by heating and re-processing. Sulfidic cross-linking represents a significant problem in recycling of rubber vulcanizate and in the recovery of the starting material rubber polymer from vulcanized rubber.
A review of methods for devulcanizing rubber vulcanizate is presented in "Methods of Devulcanization" (Warner, Walter C. Rubber Chem Technol 67:3:559-566;1994). The reviewed methods include catalysis with a quaternary ammonium chloride catalyst having a large hydrocarbon radical attached to nitrogen; grafting ethyl acrylate onto ground polybutadiene-vulcanizate waste; dissolving rubber in o-dichlorobenzene with 2,2A'-dibenzamidodiphenyl disulfide; applying microwave energy at a specified frequency and energy level; subjecting rubber to ultrasonic waves; and biodegradation with thermophilic bacteria. Chemical probes for reacting with sulfidic cross-links are also discussed and include triphenylphosphine and sodium di-n-butyl phosphite, propane-thiol/pipefidine, dithiothreitol, lithium aluminum hydride, aniline-sodium solution, phenyl lithium in benzene and methyl iodide.
Many of these prior art processes require the use of a digester and/or require stirring for many hours. Other disadvantages include expensive reagents, inefficient reactions and uneconomic processes. Other reactions may involve pyrolysis which will remove the sulfur but the polymer is broken down as a result.
It is therefore desirable to provide a process which can efficiently devulcanize a rubber vulcanizate matrix to recover the original polymeric constituents, as well as other tire constituents, such as carbon black.
It is an object of the present invention to selectively remove sulfur cross-links found in rubber vulcanizate to recover a reusable rubber polymer.